1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus having the same, and more particularly to an image forming apparatus having a sheet conveying apparatus which can detect a leading end position of a sheet to be conveyed.
2. Description of the Related Art
In general, the image forming apparatus provides a sheet conveying portion with a sheet detection portion detecting a leading end position of a sheet in order to match the time to send the sheet to an image transfer position with the time to send an image (toner image) to the image transfer position. The image forming apparatus provides the sheet conveying portion with a plurality of sheet detection portions to detect a sheet conveying state along a sheet conveying path such as a sheet conveyance delay and a jam (see Japanese Patent Application Laid-Open No. H09-183539).
FIGS. 30 to 31B illustrate a conventionally general sheet detection portion. As illustrated in FIG. 30, a conventional sheet detection portion includes a sensor flag 523 and an optical sensor 524. The sheet detection portion is arranged on a downstream side in a sheet conveying direction of a sheet conveying roller pairs 518, 519 closest to an image transfer position. The sensor flag 523 includes a rotating shaft 527 rotating the sensor flag 523; a light shielding portion 525 shielding an optical path L from a light emitting portion to a light receiving portion of the optical sensor 524; a stopper portion 526 positioning the sensor flag 523 to a home position; and a return spring 528. Even if the sensor flag 523 rotates, the sensor flag 523 returns to the home position by its own weight or a pressing force of the return spring 528.
As illustrated in FIG. 31A, when a leading end of a sheet S contacts the sensor flag 523, the sensor flag 523 rotates from the home position to a direction indicated by an arrow M1 around the rotating shaft 527 and the light shielding portion 525 shields the optical path L of the optical sensor 524. When the optical sensor 524 detects that the optical path L is shielded, the sheet detection device recognizes that the leading end of the sheet S reaches the sensor flag 523. FIG. 31B illustrates a state in which the sheet is passing through and in contact with the sensor flag 523. When a trailing end of the sheet S passes through the sensor flag 523, the sensor flag 523 returns to the home position illustrated in FIG. 31A. At this time, the light shielding portion 525 retracts from the optical path L, allowing the light receiving portion of the optical sensor 524 to receive light emitted from the light emitting portion again. Then, the sheet detection device recognizes that the trailing end of the sheet S has passed through the sensor flag 523. In recent years, the image forming apparatus has been required to meet user demand to further improve throughput. In order to improve throughput of the image forming apparatus, it is needed to increase a sheet conveying speed or shorten an interval from the trailing end of a preceding sheet to the leading end of a subsequent sheet (hereinafter referred to as a sheet gap). Consequently, the sheet detection device needs to return the sensor flag to the home position in a short sheet gap after the preceding sheet has passed
The conventional sensor flag 523 operates such that when the leading end of the sheet S passed through the conveying roller pair abuts against an abutting portion, the sensor flag is pushed by the sheet S to rotate, and when the trailing end of the sheet moves away from the abutting portion, the sensor flag reversely rotates to return to a home position P. Consequently, the distance required for the sheet gap is a distance D3 obtained by adding a distance D1 from a position in which the trailing end of the preceding sheet passes through the abutting portion of the sensor flag to the home position P in which the leading end of the subsequent sheet abuts against the abutting portion to a distance D2 between which the subsequent sheet is conveyed (see FIG. 31B).
The distance D2 is a distance obtained by multiplying a time Δt during which the sensor flag 523 moves across the distance D1 by a sheet conveying speed V (Δt×V). When the sensor flag 523 reciprocates, the distance D1 for the sensor flag 523 to return to the home position P is needed, and the higher the sheet conveying speed, the longer the distance D2 for the subsequent sheet to be conveyed during the return movement. Thus, the conventional sheet detection device has a problem in that an increase in the sheet conveying speed increases the sheet gap distance, which inhibits further improvement in throughput.